1. Field of the Invention
This invention relates to systems for, and methods of, providing for the transmission and reception of signals through unshielded twisted pairs of wires within a communications system. The invention particularly relates to systems for, and methods of, reducing the noise present in the signals received and processed by devices within a communications system and to systems for, and methods of, reducing such noise in communications systems having high throughputs. A xe2x80x9chigh throughputxe2x80x9d as used within the context of this disclosure may include, but is not limited to, one gigabit (GB) per second.
2. Description of Related Art
A basic communications system is illustrated in FIG. 1. The system includes a hub and a plurality of computers serviced by the hub in a local area network (LAN). Four computers are shown by way of illustration but a different number of computers may be contained within the system. Each of the computers is usually displaced from the hub by a distance which may be as great as approximately one hundred meters (100 m.). The computers are also displaced from each other. The hub is connected to each of the computers by a communications line. Each communication line includes unshielded twisted pairs of wires or cables. Generally, the wires or cables are formed from copper. Four unshielded twisted pairs of wires are provided in each communication line between each computer and the hub. The system shown in FIG. 1 is operative with several categories of unshielded twisted pairs of cables designated as categories 3, 4 and 5 in the telecommunications industry. Category 3 cables are the poorest quality (and lowest cost) and category 5 cables are the best quality (and highest cost).
Associated with each communications system is a xe2x80x9cthroughputxe2x80x9d. The throughput of a system is the rate at which the system processes data and is usually expressed in bits/second. Most communications systems have throughputs of 10 megabits (Mb)/second or 100 Mb/second. A rapidly evolving area of communications system technology enables 1 Gb/second full-duplex communication over existing category-5 unshielded twisted pair cables. Such a system is commonly referred to as xe2x80x9cGigabit Ethernet.xe2x80x9d
A portion of a typical Gigabit Ethernet is shown in FIG. 2. The Gigabit Ethernet provides for transmission of digital signals between one of the computers and the hub and the reception of such signals at the other of the computer and the hub. A similar system can be provided for each of the computers The system includes a gigabit medium independent interface (GMII block which receives data in byte-wide format at a specified rate, for example 125 MHz, and passes the data onto the physical coding sublayer (PCS) which performs scrambling, coding, and a variety of control functions. The PCS encodes bits from the GMII into 5-level pulse amplitude modulation (PAM) signals. The five signal levels are xe2x88x922, xe2x88x921, 0, +1, and +2. Communication between the computer and hub is achieved using four unshielded twisted pairs of wires or cables, each operating at 250 Mb/second, and eight transceivers, one positioned at each end of a unshielded twisted pair. The necessity of full-duplex bidirectional operation dictates the use of hybrid circuits at the two ends of each unshielded twisted pair. The hybrid controls access to the communication line, thereby allowing for full-duplex bidirectional operation between the transceivers at each end of the communications line.
A common problem associated with communications systems employing multiple unshielded twisted pairs and multiple transceivers is the introduction of crosstalk and echo noise or impairment signals into the transmission signals. Noise is inherent in all such communications systems regardless of the system throughput. However, the effects of these impairment signals are magnified in Gigabit Ethernet. Impairment signals include echo, near-end crosstalk (NEXT), and far-end crosstalk (FEXT) signals. As a result of these impairment signals the performance of the transceivers, particularly the receiver portion, is degraded.
NEXT is an impairment signal that results from capacitive coupling of the signals from the near-end transmitters to the input of the receivers. The NEXT impairment signals encountered by the receiver in transceiver A are shown in FIG. 3. The crosstalk signals from transmitters B, C, and D appears as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to NEXT impairment signals. For clarity of FIG. 3, only the NEXT impairment experienced by receiver A is illustrated.
Similarly, because of the bidirectional nature of the communications systems, an echo impairment signal is produced by each transmitter on the receiver contained within the same transceiver as the transmitter. The echo impairment signal encountered by the receiver in each transceiver is shown in FIG. 4. The crosstalk signals from transmitters appear as noise to the receivers, which are attempting to detect the signal from the transmitter at the opposite end of the communications line. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to the echo impairment signal.
Far-end crosstalk (FEXT) is an impairment that results from capacitive coupling of the signal from the far-end transmitters to the input of the receivers. The FEXT impairment signals encountered by the receiver in transceiver A are shown in FIG. 5. The crosstalk signals from transmitters F, G, and H appears as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to the FEXT impairment signal. For clarity of FIG. 5 only the FEXT impairment experienced by receiver A is illustrated.
As a result of these noise impairment signals the performance of the communication system is degraded. The signals carried by the system are distorted and the system experiences a higher signal error rate. Thus there exists a need in the art to provide a method of, and an apparatus for, compensating for the degradation of communication system performance caused by noise impairment signals and to provide a method of, and apparatus for, reducing such noise in a high throughput system such a Gigabit Ethernet. The present invention fulfills these needs.
Briefly, and in general terms, the invention relates to systems for, and methods of, reducing the noise present in the signals received and processed by devices within a communications system and to systems for, and methods of, reducing such noise in communications systems having high throughputs.
In one embodiment, the invention is a communications system including a communication line having a plurality of twisted wire pairs and a plurality of transmitters, one transmitter at each end of each twisted wire pair. The system also includes a plurality of receivers, one receiver at each end of each twisted wire pair. Each receiver receives a combination signal including a direct signal from the transmitter at the opposite end of the twisted wire pair with which the receiver is associated and a plurality of far-end crosstalk (FEXT) impairment signals, one from each of the remaining transmitters at the opposite end of the communications line. Further included in the system are a plurality of FEXT cancellation systems, one associated with each receiver; each FEXT cancellation system provides a replica FEXT impairment signal. The system also includes a plurality of devices, one associated with each receiver; each device is responsive to the combination signal received by the receiver and the replica FEXT impairment signal provided by the FEXT cancellation system associated with the receiver for substantially removing the FEXT impairment signals from the combination signal.
By providing a plurality of FEXT cancellation devices which generate replica FEXT impairment signals and a plurality of device which combine the replica FEXT impairment signals with the combination signals, the invention substantially cancels the FEXT impairment signals from the combination signal. Thus signal degradation due to noise in the communications system is reduced and the transmitted information may be more reliably recovered.
In another embodiment, the invention is a method for reducing noise in a communications system. The communications system comprises a communication line having a plurality of twisted wire pairs, a plurality of transmitters, one transmitter at each end of each of the twisted wire pairs, a plurality of receivers, one receiver at each end of each of the twisted wire pairs. Each receiver receives a combination signal including a direct signal from the transmitter at the opposite end of the twisted wire pair with which the receiver is associated and a plurality of far-end crosstalk (FEXT) impairment signals, one from each of the remaining transmitters at the opposite end of the communications line. The communications system further comprises a plurality of combining devices, one associated with each receiver. The method comprises the step of generating a replica FEXT impairment signal. Also included is the step of delaying the arrival of the combination signal, at the combining device, by an amount substantially equal to the time delay between the arrival, at the receiver, of the direct signal and the FEXT impairment signals. Further included is the step of subtracting the replica FEXT impairment signal from the combination signal to produce an output signal substantially devoid of FEXT impairment signals.
In yet another embodiment, the invention is a system comprising a communication line having a plurality of twisted wire pairs and having a transmitter and a receiver connected at each end of each twisted wire pair to transmit and receive sequences of signals. The system also includes a decoder responsive to the sequences of the signals from the receivers at the same ends of the twisted wire pairs for determining whether such sequences of signals are acceptable and for processing such sequences of signals in accordance with such determinations.
In an additional facet of this embodiment of the invention the decoder determines whether the sequences of the signals at the receivers are acceptable or unacceptable based on a standard system code. The code introduces constraints in the sequences of symbols that can be transmitted in such a way as to increase the minimum distance between two arbitrary allowable sequences. This increased minimum distance results in a reduced probability of error. The decoder passes acceptable sequences of signals and rejects unacceptable sequences. Acceptable sequences are those sequences which obey the code constraints while unacceptable sequences are those which violate the code constraints.
These and other aspects and advantages of the present invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings which illustrate, by way of example, the preferred embodiments of the invention.